The present invention relates to an active matrix-type liquid crystal display device having a high holding rate using a polymer dispersion-type liquid crystal display mode.
The TN-type and STN-type display elements using nematic liquid crystals have been practically used as the display element utilizing a photoelectric effect. Also the display elements using highly dielectric liquid crystals have been proposed. These require a polarizing plate and an orientating treatment. On the other hand, the dynamic scattering (DS) effect and the phase conversion (PC) effect do not require the polarizing plate but utilize the scattering by the liquid crystals. However, problems have occurred in that for example the former requires the addition of ionic additives or the high driving voltage and thus it is not suitable for the active matrix LCD.
Recently, a polymer dispersion-type liquid crystal display element utilizing the scattering by the liquid crystals has been proposed as the liquid crystal display mode without requiring the polarizing plate and the orientating treatment (refer to Japanese Patent Appln. KOKAI No. 55-96922).
It has been disclosed in for example Japanese Patent Appln. KOHYO No. 58-501631 that nematic liquid crystals having a positive anisotropy of dielectric constant are mixed in an aqueous solution of polyvinyl alcohol to obtain an emulsion with liquid crystals dispersed and the obtained emulsion is applied to a substrate having a transparent electrode followed by evaporating water to produce a film with the liquid crystals having a positive anisotropy of dielectric constant dispersed in a PVA film and then a substrate having a transparent electrode is stuck to the resulting film to obtain a liquid crystal display element.
In addition, it has been disclosed in Japanese Patent Appln. KOHYO No. 61-502128 that an epoxy resin-setting agent and liquid crystals are blended to be dissolved and the resulting mixture is put between a pair of substrates provided with a transparent electrode followed by heating to set said epoxy resin, whereby obtaining a liquid crystal display element with liquid crystals having a positive anisotropy in dielectric constant dispersed in the epoxy resin.
Also the general polymers, such as acrylic resins and urethane resins, have been known as the transparent polymers for dispersing the liquid crystal in addition to the above described epoxy resins and also liquid crystal display elements using these polymers have been proposed.
In the elements with liquid crystals having a positive anisotropy in dielectric constant dispersed in these polymers, said liquid crystals are orientated at random and a light incident upon the element is scattered on a boundary of the liquid crystal and a polymer to make the liquid crystal cloudy, whereby reducing a transmissivity of the liquid crystal, due to said anisotropy in dielectric constant of the liquid crystals when no voltage is applied. Upon applying a voltage to the element, the liquid crystals are orientated due to the positive anisotropy in dielectric constant thereof so that the direction of electric field may be the direction of long axis and a light incident upon the element is not scattered but transmitted through the element. That is to say, the element becomes transparent.
The element with the liquid crystals dispersed in the high molecule (polymer) has been utilized as a light-regulating glass aiming at the regulation of an incident light in intensity by applying said voltage or not applying the voltage to switch-over to the scattering condition and the transmissive condition in the above described manner.
In addition, recently, liquid crystal display devices using the above described element with the liquid crystals dispersed in the polymer in place of the TN-type liquid crystals have been actively developed as the active matrix-type with the nonlinear active element, such as switching transistor, added to the respective image elements. Although the TN-type liquid crystals are the present main current of the active matrix-type liquid crystal display devices, a problem has occurred in that two pieces of polarizing plate are required and a light is absorbed by these polarizing plates to reduce a brightness of a transmitted light to at most a half or less of that of an incident light. On the contrary, the element with the liquid crystals dispersed in the polymer does not require the polarizing plate, so that a utilization efficieny of light during the transmission can be improved and thus a bright display can be obtained.
The liquid crystal display device of this type with the switching transistor added comprises (i) a display electrode substrate provided with signal electrodes, scanning electrodes, switching transistors formed at the respective points of intersection of said signal electrodes and said scanning electrodes and display image element electrodes arranged on a first transparent insulating substrate, (ii) a counter electrode substrate provided with a counter electrode arranged on a second transparent insulating substrate so as to be positioned within a range corresponding to said display image element electrodes and (iii) a liquid crystal layer with liquid crystals having a positive anisotropy in dielectric constant dispersed in a transparent polymer interposed between said display electrode substrate and said counter electrode substrate.
An equivalent circuit of the respective display image elements is shown in FIG. 3. In addition, a wave form of a driving voltage in said equivalent circuit, that is wave forms of a signal voltage V.sub.S, a scanning voltage V.sub.G and a voltage V.sub.D applied to said liquid crystal layer, is shown in FIG. 4. As obvious also from these drawings, when said switching transistor is switched on by said voltage V.sub.G, V.sub.S is charged in the liquid crystal layer (equivalently serving as a condenser C.sub.LC) and held for a time until the switching transisitor is switched on again, so that V.sub.D is applied to the liquid crystal layer to generate a static driving and thus the similar superior display characteristics can be obtained.
However, in fact, a resistance component (equivalently serving as a resistance R.sub.LC) resulting from electrically conductive impurities coming from outside and formed by the decomposition of the liquid crystals and the like exist in the liquid crystal layer serving as said condenser C.sub.LC, as shown in FIG. 5, so that V.sub.D is discharged through the liquid crystal layer to be attenuated with the lapse of time, as shown in FIG. 5. As a result, an effective voltage applied to the liquid crystal layer is lowered. The liquid crystals show a cumulative response effect against a voltage, so that the display characteristics are dependent upon said effective voltage. The display contrast is deteriorated with a decrease of the effective voltage (a holding rate defined by a ratio of an effective value of V.sub.D to that in the case where it is supposed that said resistance component of the liquid crystal layer is infinitely high is used as a parameter for evaluating a degree of said decrease of the effective voltage).
In the conventional polymer dispersion-type liquid crystal display element, liquid crystals having an increased anisotropy in refractive index (.DELTA.n), such as cyano biphenyl family liquid crystals and cyano pyrimidine family liquid crystals, are used in order to heighten the light-scattering intensity but problems have occurred in that not only the holding rate of these liquid crystals themselves is inferior but also the holding rate is still more decreased in the case where they are dispersed in a polymer and their solution has been requested Problems to be Solved by the Invention.